Hi,
I have a Fender Deluxe Reverb guitar amp that had normal 6V6GT(glass tubes) in the output. I had recently swapped for old 6V6 metal casing tubes. Not long after, 3 of the 12AX7s went bad. What I would like to know is if switching to 6V6s from GTs could have affected tube life of other tubes, or if this is just all a coincidence?
I measured the plate voltage with the original glass tubes and got 410VDC and 170VDC on the 12AX7s. Switching for metal case 6V6s, the plate voltage dropped to 390VDC and on the 12AX7s, to 162VDC. I think a small change in plate voltage with a different set of tubes is probably normal, but is 20Volts difference out of the ordinary?
My thinking is that everything is normal and the 12AX7s going bad was just a coincidence, but thought I would check on this forum for opinions on the situation, so at to not blow more new 12AX7s needlessly. However, I do have at least ten of those old unused metal case 6V6s, so it would be nice if I could use them in the future.
So what say you? 🙂
Dan
I have a Fender Deluxe Reverb guitar amp that had normal 6V6GT(glass tubes) in the output. I had recently swapped for old 6V6 metal casing tubes. Not long after, 3 of the 12AX7s went bad. What I would like to know is if switching to 6V6s from GTs could have affected tube life of other tubes, or if this is just all a coincidence?
I measured the plate voltage with the original glass tubes and got 410VDC and 170VDC on the 12AX7s. Switching for metal case 6V6s, the plate voltage dropped to 390VDC and on the 12AX7s, to 162VDC. I think a small change in plate voltage with a different set of tubes is probably normal, but is 20Volts difference out of the ordinary?
My thinking is that everything is normal and the 12AX7s going bad was just a coincidence, but thought I would check on this forum for opinions on the situation, so at to not blow more new 12AX7s needlessly. However, I do have at least ten of those old unused metal case 6V6s, so it would be nice if I could use them in the future.
So what say you? 🙂
Dan
20 volts IS a small change. Only 5%.
yes, perfectly normal for different power tubes to draw different current, which causes B+ to rise or fall.
The power tubes have no way to affect the small tubes. Especially since we don't know what "went bad" means.
Be careful with the metal tubes, especially if your circuit uses pin 1 on the socket as a tine point, as so many Fender amps do. On the glass tube, pin 1 is not used, but on the metal tubes it connects to the shield.
In general, many of the metal tubes TEND to be more microphonic.
yes, perfectly normal for different power tubes to draw different current, which causes B+ to rise or fall.
The power tubes have no way to affect the small tubes. Especially since we don't know what "went bad" means.
Be careful with the metal tubes, especially if your circuit uses pin 1 on the socket as a tine point, as so many Fender amps do. On the glass tube, pin 1 is not used, but on the metal tubes it connects to the shield.
In general, many of the metal tubes TEND to be more microphonic.
Tubes are all +/-20%.
If nominal is 170V, then 136 or 204 would be expectable.
Actually a self-bias circuit (the 100K and 1.5K resistors) tends to reduce the effect of tube variation, so we usually find more like +/-10% in-circuit variation. Enzo says you have 5% change, "nothing" to a tube.
And dropping from 170V to 162V did not hurt the tubes. 12AX7 are run from under 50V to over 300V on the plate just fine.
410V to 390V at the main supply is slightly odd. Did you measure the 6V6es cathode current? Tubes have varied over the many decades and many lands since metal 6V6 walked the earth. (BTW, metal 6V6 is rated 315V max.....) Also a long-stored tube may be gassy, conducting more current (even a LOT more current) than it should.
I do think your small-tube "went bad" is coincidence.
If nominal is 170V, then 136 or 204 would be expectable.
Actually a self-bias circuit (the 100K and 1.5K resistors) tends to reduce the effect of tube variation, so we usually find more like +/-10% in-circuit variation. Enzo says you have 5% change, "nothing" to a tube.
And dropping from 170V to 162V did not hurt the tubes. 12AX7 are run from under 50V to over 300V on the plate just fine.
410V to 390V at the main supply is slightly odd. Did you measure the 6V6es cathode current? Tubes have varied over the many decades and many lands since metal 6V6 walked the earth. (BTW, metal 6V6 is rated 315V max.....) Also a long-stored tube may be gassy, conducting more current (even a LOT more current) than it should.
I do think your small-tube "went bad" is coincidence.
Ok, thanks for the replies.
Yes, I read about pin 1 on the metal tubes not long after I posted and will check into that. However, the amp is operating normally with the metal casing 6V6s, but will check anyways.
Thanks for the heads up on the current of old tubes. I will measure the current since I already had made a jig that allows measuring current for the purpose of biasing. What type of current values should I expect?
Dan
Yes, I read about pin 1 on the metal tubes not long after I posted and will check into that. However, the amp is operating normally with the metal casing 6V6s, but will check anyways.
Thanks for the heads up on the current of old tubes. I will measure the current since I already had made a jig that allows measuring current for the purpose of biasing. What type of current values should I expect?
Dan
The drop in plate volts means more current than before. I have 440volts @ 25ma for 11watts using some RCAs. If yours is a 1968-1979 I bet your over 35ma and ~14watts. If yours is 1964 or older you're probably good.
Great to hear jjman! I would guess it is between 1968-1979, just because they seemed really new in Sylvania boxes when I got them in the early 80s. Back then, they were used in low power AM transmitters and tubes were changed regularly. This batch was left over when the transmitters were replaced with solid state ones. 🙂
Thanks for all your replies, I should be good with that.
Cheers,
Dan
Thanks for all your replies, I should be good with that.
Cheers,
Dan
A 20V drop in Va suggests that the new tubes are pulling A LOT more current than the original set. Have you checked the idle condition? It seems bias needs to be adjusted.
Good point, nauta. I'll do that. It may be a few days before I get back to it, though.
Cheers,
Dan
Cheers,
Dan
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